Many medical devices adapted for implantation also have high power requirements and must be frequently connected to external power sources. Inductively coupled transcutaneous energy transfer (TET) systems are increasingly popular for use in connection with these high-power implantable devices. A TET system may be employed to supplement, replace, or charge an implanted power source, such as a rechargeable battery. Unlike other types of power transfer systems, TET systems have an advantage of being able to provide power to the implanted electrical and/or mechanical device, or recharge the internal power source, without puncturing the skin. Thus, possibilities of infection are reduced and comfort and convenience are increased.
TET devices include an external primary coil and an implanted secondary coil, separated by intervening layers of tissue. The primary coil is designed to induce alternating current in the subcutaneous secondary coil, typically for transformation to direct current to power an implanted device. TET devices therefore also typically include an oscillator and other electrical circuits for periodically providing appropriate alternating current to the primary coil. These circuits typically receive their power from an external power source.
Generally, the non-implanted portions of conventional TET systems are attached externally to the patient, typically by a belt, adhesive, or other fastener, such that the primary coil of the TET is operationally aligned with the implanted secondary coil. Such a configuration can be disadvantageous, however, particularly when only one attachment point is available. For example, connecting the primary coil of a conventional TET system to the same patch of skin for every charge can cause significant irritation at the attachment site. In addition, movements of the patient may alter the position of the primary coil so that it is not properly positioned over the implanted secondary coil to achieve a desired or required transfer of power. This is especially problematic during sleep, when a patient's unconscious movements in bed may move the primary coil out of alignment with the secondary coil. As a result, patients with conventional TET systems must often remain in a particular orientation when resting to avoid unintentionally disconnecting the primary coil.
Furthermore, should a patient's implanted TET system experience a component failure in the secondary coil, which can happen due to wire flex, electrical short, or introduction of a foreign object like a hypodermic needle, emergency surgery must be conducted before the implanted battery exhausts its charge. Such emergency surgeries pose serious risks to patients who require TET systems to survive.